1. Field of the Invention
The present invention relates to a charger for NiH/NiCd secondary batteries, and more particularly, to a charger through which a plurality of batteries in series connection can be charged. Moreover, a control IC (integrated circuit) and a detection circuit are utilized to detect and control every battery (battery set) in series connection.
2. Description of the Prior Art
Since portable electronic products, such as digital cameras, game players, etc. become more and more popular, a greater capacity of secondary batteries is more required. In addition, the charging time is preferably shorter and the charger's volume is preferably smaller. Therefore, a smaller and cheaper feature with shorter charging time and longer service life is the common requirements of the modern chargers.
The conventional NiH/NiCd battery chargers have their own advantages and disadvantages. However, they still can't reach the expected effect and need to be improved. These are described as follows:
Referring to FIG. 1, a conventional series charger is shown. This charger utilizes a transformer or an exchange power unit 11 to convert the power mains AC 100˜240V into DC 2˜5V. Meanwhile, the batteries B1, B2, B3, B4 . . . in series connection are charged by a charging circuit 12. This charging mode has advantages of simple configuration and low price. However, its disadvantage lies in that one is fully charged while the other is not. This will easily lead to overcharge of the full-charged battery when the charging process is continued, thereby lowering its service life.
The reason for that lies in that most of the commercially available smart chargers utilize ΔV, as shown in FIG. 3, to detect the full-charging point of the NiH/NiCd secondary battery. In other words, when one battery is fully charged (Vp), its voltage begins to reduce by ΔV. However, the voltage of the not-fully-charged battery still rises. Therefore, the summed voltage of the charged batteries in series connection won't be reduced such that the fully charged batteries will be overcharged until all batteries are fully charged. In case of abnormality of one battery, an unexpected danger could happen.
Besides, another disadvantage of the series charger lies in that single battery can't be charged. A charging loop can only be created only when all series-connected batteries to be charged are connected together.
Referring to FIG. 2, a conventional charger in parallel is shown. All parallel-connected batteries B1, B2, B3, B4 are charged by a charging circuit 12. The advantage thereof lies in that every battery has almost the same charging voltage without disadvantages of overcharge or being not-fully-charged status of the series batteries. However, the greatest disadvantage of the parallel charger lies in that it can't meet the requirements of speedy charging because the choice and the installation of the electronic control elements are difficult. Supposed that the charging voltage is 2V, and the charging current of each parallel-connected battery is 1.5 A, the required total current will be 6 A. However, the diode of the electronic components has problem with internal resistance while the transistor has problem with pressure difference. To create a low-voltage and high-current charging loop has much to be overcome. For example, it's difficult to fulfill the safety requirement that the wiring of the circuit board has to amount to over 6 mm. Thus, the volume of the parallel charger can't be reduced. Moreover, the components are easily over-heated. Furthermore, the rapid charging requirement can't be fulfilled. If the voltage is increased to enhance the power for reducing the current value, the four switch transistors Q1, Q2, Q3, Q4 will due to the pressure difference produce a high temperature which is difficult to be overcome. And to add cooling elements will increase the whole volume and the material cost.